All-solid-state batteries are known as batteries excellent in safety. As a technique relating to all-solid-state batteries, a technique of making a composite particle by forming an ion conductive oxide on the surface of an active material is known. For example, WO 2007/004590 discloses a technique of forming an LiNbO3 covering layer on the surface of an LiCoO2 powder, via a process of hydrolyzing an alkoxide solution including lithium and niobium, at the surface of an LiCoO2 powder particle. JP 2012-74240 A discloses a lithium-transition metal oxide particle whose carbon content is no more than 0.03 mass %, consisting of a lithium-transition metal oxide particle. At least a part of the surface of the lithium-transition metal oxide particle is covered by a covering layer including lithium niobate.